[mlir] LoopToStandard conversion: support "if/else" with results

Summary:
A recent extension allowed the `loop.if` operation to return results yielded by
its regions. However, such operations could not be lowered to a CFG of standard
operations because it would have required to modify the argument list of a
block, which is not allowed in a conversion pattern. Now that the conversion
infrastructure supports block creation, use it to create a block with an
argument list that dominates the operations following the `loop.if` and forward
the results as arguments of this block.

Depends On D77416

Differential Revision: https://reviews.llvm.org/D77418